Overcurrent Protection in Thyristors
Thyristors have small thermal time constants. Therefore, if a thyristor is subjected to overcurrent due to faults, short circuits or surge currents ; its junction temperature may exceed the rated value and the device may be damaged. There is thus a need for the overcurrent protection of SCRs. As in other electrical systems, overcurrent protection in thyristor circuits is achieved through the use of circuit breakers and fast-acting fuses as shown in Fig. 4.29.
The type of protection used against overcurrent depends upon whether the supply system is weak or stiff. In a weak supply network, fault current is limited by the source impedance below the multi-cycle surge current rating of the thyristor. In machine tool and excavator drives, if the motor stalls due to overloads, the current is limited by the source and motor impedances. The filter inductance commonly employed in dc and ac drives may limit the rate of rise of fault current below the multicycle surge current rating of the thyristor. For all such systems, overcurrent can be interrupted by conventional fuses and circuit breakers. However, proper co-ordination is essential to guarantee that (i) fault current is interrupted before the thyristor is damaged and (ii) only faulty branches of the network are isolated.
Conventional protective methods are, however, inadequate in electrical stiff supply networks. In such systems, magnitude and rate of rise of current is not limited because source has negligible impedance. As such, fault current and therefore junction temperature rise within a few milliseconds. Special fast-acting current-limiting fuses are, therefore, required for the protection of thyristors in these stiff supply networks.
The operation of fast-acting current-limiting fuse is illustrated in Fig. 4.27 (b). These fuses and thyristors are found to have similar thermal properties, there co-ordination is therefore simpler. The current-limiting fuse consists of one or more fine silver ribbons having very short fusing time. In Fig. 4.27 (b), fault is shown to occur at zero crossing of the ac sine wave, i.e. at t = 0, Without fuse, the fault current would rise upto A and then would follow dotted curve. A properly selected current limiting fuse melts at A. An arc is then struck. For a brief interval after A, the current continues to rise depending upon the circuit parameters and the fuse design. This current reaches a peak value, called peak let through current, which is indicated by point B in Fig. 4.27 (b). Note that peak let through current is considerably less than the peak fault current without the fuse, the latter is indicated by point D. After the point B, arc resistance increases and fault current decreases. At point C, arcing stops and the fault current is cleared. The total clearing time tc is the sum of melting time tm and arcing time ta, i.e. tc = tm + ta.
Proper co-ordination between fast-acting current-limiting fuse and thyristor is essential. A fuse carries the thyristor current as both are placed in series. Therefore, the fuse must be rated to carry full-load current plus a marginal overload current for an indefinite period. But the peak let through current of fuse must be less than the subcycle surge current rating of the SCR. The voltage across the fuse during arcing period is known as arcing, or recovery, voltage. This voltage is equal to the sum of source voltage and the emf induced in the circuit inductance during arcing time ta. If the fuse current is interrupted abruptly, induced e.m.f.
L – may be high; as a result arcing voltage would be excessive. It should therefore be ensured during fuse design and co-ordination that arcing voltage is limited to less than twice the peak supply voltage. In case voltage rating of the fuse is far in excess of circuit voltage, an abrupt current interruption would lead to dangerous overvoltages.
When both circuit breaker and fast-acting current-limiting fuse are used for overcurrent protection of SCR, Fig. 4.29, the faulty circuit must be cleared before any damage is done to the device. A circuit breaker has long tripping time, it is therefore generally used for protecting the semiconductor device against the continuous overloads or against surge currents of long duration. A fast-acting C.L. fuse is used for protecting thyristors against large surge currents of very short duration. The tripping time of the circuit breaker, the fusing-time of the fast-acting fuse must be properly co-ordinated with the rating of a thyristor. In order that fuse protects the thyristor reliably, the I2t rating of the fuse must be less than that of the SCR.